An acrylic rubber has good heat resistance and good oil resistance, and therefore, is widely used in an automobile field and related fields as a material for rubber parts which include, for example, sealers such as gasket and packing, hoses, vibration insulators, tubes and belts. Acrylic rubbers suitable for these rubber parts have a rubber elasticity formed by crosslinking an acrylic rubber having active crosslinking sites introduced therein usually by copolymerization of 1 to 5% by weight of a crosslink-forming monomer.
The choice of a crosslink-forming monomer as well as a crosslinking agent used in combination therewith materially affects a rate of crosslinking reaction, which influences storage stability, mechanical properties, permanent set, heat resistance and other properties. In general, as the crosslink-forming monomer, there can be mentioned chlorine-containing monomers such as 2-chloroethyl vinyl ether and vinyl chloroacetate, and epoxy monomers such as allyl glycidyl ether.
Crosslinked products having good physical properties can be obtained by using these crosslink-forming monomers. However, a crosslinked product obtained by copolymerization of a chlorine-containing monomer tends to cause corrosion of metal having contact with the crosslinked product.
The use of monoesters of a butenedioic acid such as maleic acid and fumaric acid as a crosslink-forming monomer has been proposed in, for example, Japanese Unexamined Patent Publication S50-45031 and ibid. H11-92614. Especially it has been reported therein that an acrylic rubber composition comprising an acrylic rubber having copolymerized therein 0.1 to 10% by weight of a mono-lower alkyl ester of fumaric acid, and an aromatic diamine crosslinking agent and a guanidine compound crosslinking aid, gives a crosslinked product causing corrosion of metal in a lesser extent and having high heat resistance. However, this acrylic rubber composition has a problem such that scorching is liable to occur during processing, especially at an initial shaping stage when extrusion shaping is conducted.